1. Field of the Invention
The present invention relates to a semiconductor device and a manufacturing method thereof, and more particularly to a semiconductor device including a metal/metal-capacitor having a capacitance value which is not dependent on an applied voltage, and being formed of the same material and by the same process as a laminated film constituting an interconnect layer, and a method for manufacturing the semiconductor device.
2. Description of the Related Art
In recent years, capacitors having various structures are used in highly integrated circuits. For example, there are known a capacitor having a structure such that a dielectric film is sandwiched between a diffusion region formed in a semiconductor substrate and a polysilicon electrode formed on the substrate, a capacitor having a structure such that a dielectric film is sandwiched between top and bottom polysilicon electrodes, and a capacitor having a structure such that a dielectric film is sandwiched between top and bottom metal electrodes.
Among these capacitors, a capacitor constituting an analogue circuit determines a magnitude of an output signal, so that variation in the capacitance value of the capacitor causes variation in the output signal. Therefore, an A-D/D-A converter for example is desired to have a capacitor structure with a negligible applied-voltage dependency of the capacitance value in order to prevent operation errors
A capacitor in which a dielectric film is sandwiched between a diffusion region and a polysilicon electrode gives rise to a PN junction capacitance between the diffusion region and a substrate. Since this PN junction capacitance has a large voltage dependency of the capacitance value, it is difficult to realize a capacitance value which is not dependent on an applied voltage.
For a capacitor in which a dielectric film is sandwiched between top and bottom polysilicon electrodes, the polysilicon electrodes must be doped heavily to have a large impurity concentration so as to reduce the resistance and the voltage dependency coefficient of the polysilicon electrodes.
However, in accordance with simplification of manufacturing processes and scale reduction of semiconductor devices, a step of doping the polysilicon is carried out simultaneously with a doping step for forming source/drain diffusion regions of a transistor. Further, a thermal treatment time is shortened to control diffusion of impurity ions for forming the source/drain regions. As a result, the doping concentration in the polysilicon electrodes is reduced, leading to insufficient reduction of the resistance and the voltage dependency coefficient of the electrodes.
On the other hand, a capacitor in which a dielectric film is sandwiched between top and bottom metal electrodes is effective especially as analogue capacitor because the capacitance of the capacitor is not dependent on the applied voltage.
For example, Japanese Unexamined Patent Publication No. HEI 05(1993)-129522 discloses a metal/metal-capacitor structure in which a top electrode 26 is formed of aluminum, and a bottom electrode 22 is formed of a high melting point metal, as shown in FIG. 4. Also, in this publication, an electrically conductive protection film 25 is formed on a dielectric film 24 side of the top aluminum electrode 26 in order to prevent formation of hillocks of the top electrode 26 caused by a thermal treatment in the manufacturing process.
Therefore, by using such a metal/metal-capacitor structure, it is possible to realize an effective capacitor in which the applied voltage dependency is improved and further a dielectric strength is improved.
Here, even if the above-mentioned metal/metal-capacitor structure is adopted, it is necessary to form the top electrode, the bottom electrode and the interconnect layer with the same material and by the same process in order to simplify the process of manufacturing the semiconductor device.
For example, an interconnect layer to be used in an integrated circuit is typically made of a laminated film of a barrier metal and an aluminum film (See Japanese Unexamined Patent Publication No. HEI 08(1996)-274172). Here, the barrier metal is formed of a TiN/Ti layer. The Ti layer serves to obtain a good electrical connection by reducing oxides between the metals, and the TiN layer serves to strengthen the resistance to electromigration (migration of metal atoms by an electric current) of aluminum. The barrier metal serves to prevent generation of a tungsten-aluminum alloy by inhibiting direct contact between tungsten constituting a tungsten plug formed under the barrier metal and aluminum formed above the barrier metal.
However, the capacitor structure shown in FIG. 4 is formed through a series of steps including formation/patterning of a capacitor bottom electrode 22 on a semiconductor substrate 20 having a device isolation film 21, formation/opening of an interlayer dielectric film 23, and formation/patterning of an electrically conductive protection film 25 and a top electrode 26. Here, an interconnect layer is not described in the prior art shown in FIG. 4. However, if an interconnect layer is to be formed, only the top electrode 26 of the capacitor can be formed of the same material as the interconnect, because only the top electrode 26 can be formed by the same process as the interconnect layer. As a result, it is not possible to form a barrier metal with a laminated film typically used in an integrated circuit.
Also, as mentioned above, in order to form the capacitor structure shown in FIG. 4, it is necessary to separately carry out a photolithography technique for forming a bottom electrode 22 of the capacitor and a photolithography technique for forming a top electrode 26 of the capacitor, thereby complicating the manufacturing process.